Concerns about the cost of operating and the environmental impact of motorized vehicles has generated heightened interest in regenerative brake systems. Typically regenerative brake systems capture and store some or all of the energy that would otherwise be dissipated as heat by a friction based braking system during deceleration of the vehicle. The stored energy can be used to accelerate the vehicle either alone or in combination with another energy source such as an engine. Vehicles equipped with a regenerative brake system and another source of motive power (e.g. an engine or motor) are often referred to as hybrid vehicles.
Many alternative implementations of hybrid vehicles have been developed using various types of regenerative brake systems; electric and hydraulic regenerative brake systems being commonly used. One aspect that many of the different hybrid vehicle implementations share is complexity. A typical hybrid vehicle can include an internal combustion engine, a mechanical transmission, an electric motor/generator, a planetary gearbox, a bank of batteries, an alternating current/direct concurrent (AC/DC) converter, a hydraulic operated friction brake system, and at least one control module to control and coordinate the operation of the other components.
Some of the complexity of a hybrid vehicle implementation can be reduced through the use of a hydrostatic brake system where one or more hydraulic pump/motors each acting on one or more axles of the vehicle are used as the primary brake system and also as the regenerative drive element. Typically the hydraulic pump/motor includes a reciprocating piston or a rotating rotor whose speed of reciprocation or rotation is a function of the speed of rotation of the vehicle axle. The power (braking and drive) transferred by the hydraulic pump/motor is a function of the flow (i.e. volume/time) and pressure of hydraulic fluid displaced through the pump/motor. In order for the pump/motor to transfer sufficient power at low rotational speeds of the axle (i.e. low shaft speeds of the pump/motor) a relatively large displacement is required by the pump/motor. At higher vehicle speeds (i.e. higher shaft speeds), the large displacement becomes problematic as it necessitates the movement of large flows of hydraulic fluid resulting in the need for a large and costly hydraulic infrastructure
What is needed is a less complex implementation of a hybrid vehicle and a hydraulic pump/motor for use therein that mitigates the need for moving large flows of hydraulic fluid at high vehicle speeds.